Flooding concerns rise for southern Finger Lakes

finger lakes weather forecast wednesday december 23 2020 flood watch flash flood christmas eve rain wind flooding
A combination of prolonged warm, moist winds, snow melt, and heavy rain will lead to areas of flooding Thursday afternoon and evening. [Photo by Nanci McCraine]

Rain & Snow Melt

A prolonged period of warm, moist wind and heavy rain will lead to rapid snow melt and flooding potential Thursday afternoon and Thursday night.

A large storm system over the Midwest today will spawn a second low pressure as it tracks east. Out ahead of this complex, warm, moist air will surge north from the Gulf of Mexico into our region.

For today, the only impacts from this complex will be periods of cloud cover. Some of the clouds may be thin enough to allow filtered sun to shine through, but these occasions should become fewer as the day goes on.

Winds will be from the south today and will gradually increase in speed. By this afternoon, wind speeds will be 5-10 mph, and by tonight, 10-15 mph. Wind gusts will start to go over 30 mph by sunset, and could reach as high as 40 mph overnight, mostly over higher elevations.

High temperatures today will range through the low 40s and upper 30s. After sunset, temperatures will only drop a couple degrees before holding steady through the night.

Atmospheric moisture will slowly increase overnight before making a bigger jump tomorrow morning. By midday, the moisture in the atmosphere will be over two-and-a-half times greater than normal, and by evening, three to four times greater than normal.

Meanwhile, winds will continue to gust as high as 40 mph throughout Thursday as temperatures rise into the mid and upper 40s and low 50s. The warm, moist, persistent wind on its own will lead to rapid snowmelt.

Rain will increase during the midday hours Thursday and quickly become heavy at times. Periods of heavy rain will continue into the evening and deep into the overnight. Widespread rain amounts over an inch are expected, especially across the southern half of the region. In addition, the areas that saw over a foot of snow fall last week have 2-4 inches of additional liquid locked up in the snowpack, most of which will melt.

finger lakes weather 7-day forecast wednesday december 23 2020
This graphic represents an average over the entire Finger Lakes region. Localized variations should be expected. Click to enlarge.

Flash Flooding and Flash Freezing

Urban and flash flooding will be possible, especially in areas where storm drains are blocked by snow. Small streams will quickly rise Thursday and could lead to localized flooding. Even the main stem rivers in the Southern Tier could go over flood stage Thursday night into Friday.

This is an unfortunate alignment of prolonged warm, moist winds, heavy rain, and a deep snowpack that may make for a difficult and at times dangerous Christmas Eve for the same areas that were hit hardest last week. A change over to snow will occur from west to east between roughly 1-7 AM Friday morning. A brief period of ice may accompany the changeover, as well as black ice formation due to plunging temperatures.

As the front responsible for the change over moves through, temperatures will drop from near 50 to near 30 in an hour or less. A slower but steady drop back into the 20s is expected thereafter.

Most of the precipitation will outrun the cold air, so most of the Finger Lakes is only looking at a coating to perhaps an inch of snow. That may make travel early Friday morning extra slick, though.

Winds will be much slower behind the front from the west. Snow showers will be possible, but with little real accumulation. Heavy lake effect will develop just outside of the region, though. Travel south of Buffalo and in the Watertown area will be significantly impacted by these lake snows.

Snow showers will continue Saturday as the lake effect weakens and moves south into our region. Sunday should be quiet with sun transitioning to clouds. High temperatures Saturday will be in the 20s, but Sunday should break back into the 30s.

More Information:

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finger lakes weather 7-day forecast wednesday december 23 2020This graphic represents an average over the entire Finger Lakes region. Localized variations should be expected.

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Follow Meteorologist Drew Montreuil:
Meteorologist Drew Montreuil has been forecasting the weather in the Finger Lakes region since 2006 and has degrees in meteorology from SUNY Oswego (B.S. with Honors) and Cornell (M.S.). Drew and his wife have four young boys. When not working or playing with the boys, he is probably out for a run through the countryside.